1. Field of the Invention
The present invention relates to an adaptive digital filter capable of iteratively updating coefficient parameters of the adaptive digital filter in response to an error signal corresponding to a difference between output signals available from an unknown system and the adaptive digital filter thereby identifying a transfer function of the adaptive filter with that of the unknown system.
2. Description of the Prior Art
Such an adaptive filter is employed as a howling preventing device in a teleconferencing system and an echo canceller in a telephone system, and disclosed, for example, in "An Acoustic Echo Canceller for Teleconference" by Yasuo Itoh et al, IEEE International Conference on Communication pp 1498-1502, 1985.
An adaptive digital filter typically is of a recursive type. A prior adaptive filter of this type is known which has a transfer function H(Z) expressed by an equation (1) and adaptively adjusts coefficient parameters a.sub.0i, a.sub.1i, a.sub.2i in response to an error signal and thereby estimates a transfer function H.sub.0 (Z) of a unknown system. ##EQU1## where Z is an operator in the Z-transformation. The coefficient parameters b.sub.1i, b.sub.2i in the denominator of the equation (1) are previously set to an average value of the unknown system usually evaluated by another measurement, and are rarely adaptively adjusted using a stability determination circuit.